CVE-2026-93642

Executive Summary

CVE-2026-93642 is a stored cross-site scripting (XSS) vulnerability in the Zimbra Modern web interface, triggered when a signed-in recipient clicks "Accept Share" on a forged share notification delivered by an unauthenticated sender. Because the injected script executes in the context of the victim's authenticated mailbox session, the attacker can read mailbox data and perform actions as the victim. With a CVSS 3.1 base score of 9.3 (Scope: Changed, Confidentiality and Integrity: High), this is a critical-severity issue for any organization exposing the Zimbra Modern web client; no patch, workaround, public PoC, or vendor fix is currently recorded by tracked sources.

Technical Analysis

The vulnerability class is stored cross-site scripting (CWE-79). According to the NVD description, the flaw resides in the handling of share notifications within the Zimbra Modern recipient interface. An unauthenticated sender can construct and deliver a forged share notification that, when a signed-in recipient clicks the "Accept Share" action, causes attacker-controlled script content to be persisted and then rendered in the victim's browser session.

Key structural facts derivable from the description:

1. Delivery primitive: The attacker does not need credentials. The share notification can be forged and sent by an "unauthenticated sender," meaning the attack surface begins at the mail/notification ingestion path rather than requiring prior mailbox access.
2. Persistence/render point: The payload is described as stored XSS, so the malicious content is retained (e.g., in the share object or the notification content) and rendered later rather than reflected immediately. Execution is deferred until the recipient confirms with "Accept Share."
3. Interaction requirement: Exploitation requires the recipient to click "Accept Share." This is reflected in the CVSS vector (UI:R), which reduces exploitability slightly but does not reduce the high confidentiality and integrity impact.
4. Scope change: The vector includes S:C (Scope: Changed). This indicates the vulnerable component's security authority is exceeded — the script is not confined to the share-notification origin but gains capabilities in the broader authenticated application context, consistent with the stated outcome of accessing mailbox data and acting as the victim.
5. Client-side execution: Because the impact is delivered through the victim's browser session, the attacker inherits the victim's session cookies, CSRF tokens, and mailbox permissions in the Zimbra Modern client. No server-side authentication bypass is described.

The description does not disclose the specific parameter, DOM sink, or API endpoint used to persist the payload, and no CVE metadata provides affected version strings. Those implementation specifics should not be assumed. What is certain from the source data is the mechanism: unauthenticated share-notification forgery leading to stored XSS on the "Accept Share" click path in the Zimbra Modern client.

Exploitation Scenarios

No public PoC is available; the chain below is inferred strictly from the vulnerability class (stored XSS via a forged notification) and the NVD description. No specific endpoints, parameters, or function names are asserted.

1) Reconnaissance / target identification The attacker identifies an organization running the Zimbra Modern web client — for example, via Shodan or by observing Zimbra login pages during routine external scanning. The attacker then needs only a valid target address to which a share notification can be delivered, since the sender is unauthenticated.

2) Triggering the flaw class (notification forgery) The attacker crafts a share notification whose content carries attacker-controlled markup. Because the sender is unauthenticated, the notification does not require a legitimate account and can be sent directly to the victim. At the vulnerability-class level, this is a classic stored-XSS injection: untrusted input is accepted into a share/notification object.

3) Execution on user interaction The payload remains dormant until the signed-in recipient reads the notification and clicks "Accept Share." Per the description, this action is what causes the stored script to execute in the Zimbra Modern context. The CVSS UI:R rating confirms that user interaction is a precondition; the attacker must induce the click, e.g., through a plausible-looking share of a document, calendar, or folder.

4) Post-exploitation impact Because the script runs with the security context exceeding the notification (S:C), the attacker operates inside the victim's authenticated session. The description states the outcome plainly: access to mailbox data and the ability to act as the victim. In class terms, this typically means reading mail, exfiltrating content, and issuing mailbox actions under the victim's identity while their session remains active — without needing the victim's password and without server-side authentication bypass.

There is no evidence of weaponization, exploit code, or in-the-wild use in the provided data.

Impact Assessment

CVE-2026-93642 is rated CVSS 3.1 9.3 (Critical) with the vector CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:N.

• AV:N (Network): Exploitable remotely; no local access to the Zimbra server or client is needed.
• AC:L (Low): No special conditions beyond delivering the notification and obtaining a click.
• PR:N (None): The sender requires no prior account or privileges — the notification can be forged by an unauthenticated party.
• UI:R (Required): The victim must click "Accept Share," which is the only meaningful friction and the primary detection opportunity.
• S:C (Changed): The compromise crosses from the notification into the broader Zimbra Modern session, materially raising risk.
• C:H / I:H: Full loss of confidentiality and integrity within the victim's mailbox context as described ("access mailbox data and act as the victim").
• A:N (None): No availability impact is described.

Business impact: An attacker who successfully lands this attack can read sensitive email (credentials resets, business communications, M&A material, customer PII) and send or modify messages as the victim, enabling business email compromise, internal phishing, lateral movement via trusted internal mail, and data exfiltration. Because the triggering notion is an ordinary "share" prompt to a legitimate signed-in user, user awareness alone is an unreliable control. The blast radius is every mailbox reachable through a vulnerable Zimbra Modern deployment for which the victim's session can be exposed — particularly high-value targets (executives, IT administrators, finance). Blast radius cannot be bounded to specific versions because no affected version strings are published in the source data.

Detection Guidance

Sigma Rule

Detects the server-side or browser-side indicators of the stored-XSS click path — i.e., a share acceptance immediately followed by anomalous script execution or session activity. Tune the share-notification event source to your Zimbra deployment's logging.

title: Zimbra Modern Share Acceptance Followed by Suspicious Script Execution
id: 4b9c2f1e-7a3d-4e21-9c88-cve202693642
status: experimental
description: Detects a Zimbra Modern "Accept Share" action followed by script execution or anomalous session activity consistent with stored XSS abuse.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-93642
author: Valters IT Threat Intelligence
date: 2026/01/01
logsource:
  product: zimbra
  service: mailbox
detection:
  selection_share:
    event_type:
      - 'share_accepted'
      - 'notification_accept'
    action: 'AcceptShare'
  selection_anomaly:
    - js_execution: true
    - url_contains: 'javascript:'
    - script_tag_in_payload: true
  condition: selection_share and selection_anomaly
falsepositives:
  - Legitimate rich HTML shares from trusted senders
level: high
tags:
  - attack.t1204.001
  - attack.t1059.007
  - cve.2026.93642

YARA Rule

Targets stored-XSS payload artifacts persisted in Zimbra share/notification storage (exported blobs, mailbox dumps, or message store objects). Hunt with this against store exports, not live network traffic.

rule CVE_2026_93642_Zimbra_StoredXSS_Share
{
    meta:
        description = "Detects stored XSS payload artifacts associated with forged Zimbra share notifications (CVE-2026-93642)"
        author = "Valters IT Threat Intelligence"
        date = "2026-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-93642"
        cve = "CVE-2026-93642"
        severity = "critical"

    strings:
        $share1 = "Accept Share" ascii nocase
        $share2 = "AcceptShare" ascii nocase
        $s1 = "<script" ascii nocase
        $s2 = "javascript:" ascii nocase
        $s3 = "onerror=" ascii nocase
        $s4 = "onload=" ascii nocase
        $s5 = "<iframe" ascii nocase
        $s6 = "document.cookie" ascii nocase

    condition:
        2 of ($s*) and 1 of ($share*)
}

Suricata IDS Rule

Detects share-notification traffic carrying executable script payloads in the request/response body. Adjusted to the class-level pattern (script markup inside share/notification content).

alert http any any -> any any (msg:"CVE-2026-93642 Zimbra Modern Forged Share Notification Stored XSS Attempt"; flow:established,to_server; http.method; content:"POST"; http.uri; content:"Share" nocase; http.request_body; content:"<script"; nocase; nocase; http.request_body; content:"Accept"; nocase; sid:9364201; rev:1; reference:cve,2026-93642; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-93642; classtype:web-application-attack; metadata:service http;)

Elastic Detection

EQL hunting query for a Zimbra share-accept event immediately followed by JavaScript execution or abnormal session operations within a short window.

sequence by user.name with maxspan=30s
  [ any where event.category == "web" and
        (url.path : "*Share*" or url.path : "*share*") and
        http.request.method == "POST" ]
  [ any where event.category == "process" and process.name : ("chrome", "firefox", "msedge") and
        process.command_line : "*javascript*" ]

Splunk SPL Query

Correlates Zimbra web access logs where a share acceptance is followed by script-like query/body patterns from the same client IP and user.

index=zimbra OR index=web_proxy sourcetype=zimbra:access OR sourcetype=access_combined
| eval is_share = if(match(uri, "(?i)share") AND match(uri, "(?i)accept"), 1, 0)
| eval is_xss   = if(match(uri, "(?i)(%3Cscript|javascript:|onerror=|onload=|<script)"), 1, 0)
| where is_share=1 OR is_xss=1
| stats count values(uri) as uris by src_ip user
| where count > 1
| sort - count

Wazuh OSSEC Rule

Detects the stored-XSS patterns in Zimbra web logs/IDS output.

<group name="web,zimbra,cve-2026-93642,">
  <rule id="936420" level="12">
    <if_sid>31100</if_sid>
    <match>Accept Share</match>
    <regex>(?i)(\&lt;script|javascript:|onerror=|onload=)</regex>
    <description>CVE-2026-93642: Possible Zimbra Modern stored XSS via forged share notification on Accept Share</description>
    <mitre>
      <id>T1204.001</id>
      <id>T1059.007</id>
    </mitre>
    <group>cve_2026_93642,web_attack,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 — UNPATCHED. No patch, vendor fix, or workaround exists in any tracked source, and no affected version list is published. Treat all internet-exposed Zimbra Modern deployments as potentially vulnerable until the vendor ships an advisory.

1) Immediate mitigations

• Restrict or disable external share-notification delivery to Zimbra Modern users where operationally possible (block share notifications from external/unauthenticated senders at the MTA or collaboration layer).
• Enforce a strict Content-Security-Policy on the Zimbra web client, disallowing unsafe-inline and inline script execution, to reduce XSS impact.
• Deploy the Suricata and Wazuh detections above to alert on script markup in share/notification content and on "Accept Share" followed by anomalous activity.
• User guidance: treat unsolicited share notifications with heightened suspicion; do not click "Accept Share" on unexpected shares. Note that user training is a soft control only.
• Reduce session exposure: require short session lifetimes and re-authentication for sensitive mailbox actions.

2) Patch / upgrade steps

• No patch exists. Monitor the Zimbra security advisory channel and NVD entry for CVE-2026-93642; apply the vendor fix immediately upon release and inventory all Zimbra Modern instances in the meantime.
• If a patch is published, prioritize internet-facing and externally reachable web clients first.

3) Long-term hardening

• Enforce output encoding and contextual sanitization of all share/notification content at render time as a defense-in-depth control.
• Adopt a CSP with nonce/hash-based script policies across the whole web client.
• Segment and monitor Zimbra infrastructure; alert on mailbox access deviations (new device, unusual geography) that could indicate session abuse.
• Maintain rapid patch-tracking for Zimbra and inventory external-facing collaboration portals.

References

• NVD — CVE-2026-93642: https://nvd.nist.gov/vuln/detail/CVE-2026-93642
• MITRE CWE-79 — Improper Neutralization of Input During Web Page Generation (Cross-site Scripting): https://cwe.mitre.org/data/definitions/79.html
• MITRE ATT&CK T1204.001 — User Execution: Malicious Link: https://attack.mitre.org/techniques/T1204/001/
• MITRE ATT&CK T1059.007 — Command and Scripting Interpreter: JavaScript: https://attack.mitre.org/techniques/T1059/007/
• Zimbra Security Center (vendor advisories — no advisory currently published for this CVE): https://wiki.zimbra.com/wiki/Security_Center

No public proof-of-concept, exploit code, or exploit-in-the-wild evidence is available for CVE-2026-93642 in the provided data, and it is not listed in CISA KEV. No threat-actor campaign attributions can be made.